Metal ducts, especially at high speeds the air vibrate and create additional noise. This fact should be borne in mind for the "quiet" production, as well as for buildings, which houses laboratories for the engineering corps, etc. There are instances where due to noise caused by vibration of metal ducts, it is impossible to exploit forced ventilation.
If you have to use metal ducts of large cross section (usually air supply), then they should perform hard and isolating porous material. To create a rigidity to be mounted air ducts in accordance with SNIP III-D. 1-62 out of steel no less than 1 mm and install the ribs of the steel angle at a distance of 200-300 mm apart. Outside air ducts are insulated slag wool, Mounted to the ribs, and plaster.
Silencers on air handling systems - a common phenomenon, not causing any problems. You only need a place to install them. Silencers are used chamber, cellular, and other labyrinthine structures. They are fairly well covered in the literature.
Contrast this with the exhaust systems. Here, in the case of non-high sound pressure levels have to install a silencer on the air outlet into the atmosphere, ie, after the fan. If the emitted air does not contain any substances liable to be deposited on the walls of the muffler, then the installation itself, in general, possible. The silencer can be of any type. In particular, the possible use of a cellular silencer,
Practical information on silencing PA pressure side exhaust systems not well understood.
Keep in mind that the noise from ventilation systems na industrial plant there when the fans are located in the working environment or outside the building, near the windows opened. If the fan is not covered with soundproof enclosure, the noise is distributed in the room through the open window in a warm period.